Dimensional accuracy is the precision with which machining of a given work piece is comparable to some basic dimensions. Accuracy shows how closely an output conforms to allowable tolerance. Normally, the tolerance is within some specified range. Dimension accuracy varies for different works; hence, a designer needs to know the best procedure that would fit a given output.
Surface Finish Parameters
Different operations are employed to improve characteristics of surface roughness. The most common surface finish parameters are Roughness average (Ra) and Root-mean-square (Rq).
Roughness average (Ra) is also referred to as Center Line Average (CLA), arithmetic mean roughness, or arithmetic average (AA). It is common and has the following expression:
Where Ra is the deviation of the arithmetic mean from the mean line, L is the distance the surface roughness is measured, and Y is the vertical deviation from the nominal surface.
In the above formula, L can be taken to be an arithmetic mean signifying departure of the roundness profile from the mean line.
Maximum peak-to-valley roughness height (Ry or Rmax) is the distance between two parallel lines with main line that contacts the extreme points on the profile within the sampling length.
Roughness average (Ra) was selected for use in the project to calculate the effect of tool geometry on surface roughness.
Diameter Error
Diameter error is the difference between measured and designed diameter of a component. It is commonly noticed in the cutting process for cylindrical components. Considerations should be made to avoid the error in the cutting process.
Circularity
Circularity is a characteristic which shows roundness of parts. It is considered an important feature that affects the dimensional accuracy of a work piece. Circularity can be examined using signal to noise ratio values.